In 1977, as already mentioned, two papers (Nelson et al., 1977 and Bennett et al., 1977) were published simultaneously in Science, reporting on the development of such a method, which added a particle accelerator into the mass spectrometer to produce an accelerator mass spectrometer.
This technique has allowed the measurement of radiocarbon in samples of much less than a milligram, or more than a thousand times less material than is needed for the older counting methods.
We can equally well use a different standard if we know its relation to "modern," or 1950 AD.
The practical use of accelerator mass spectrometry was shown in 1977 by two groups simultaneously at Mc Masversity and at the universities of Toronto and Rochester (N. The great advantage of using AMS is that we can measure the isotope ratio of C to stable carbon directly.
The number of applications of AMS today is large, and so we will focus on a general overview of some interesting applications that will give some flavor for the variety of uses of the method.
C produced in the atmosphere were always the same, then we could calculate a "radiocarbon age" using the equation we have discussed directly as an estimate of sample age. This was recognized soon after Libby published his first Curve of Knowns (Arnold and Libby, 1949).
The cosmic rays striking the upper atmosphere fluctuate in intensity with time by a small amount due to changes in the magnetic fields of the sun and the earth.
This creates an error in the "raw" age of about 2 percent.
Since nearly all applications where the precise age is needed require calibration, this difference is removed in the calibration process].These methods relied on the observation of a decay of the radioactive carbon atoms.When a C atom decays, it emits a beta particle, which can be counted in a gas by the electrical pulse it generates.Libbys measurements on C, using samples of several grams of carbon-black powder (see Anderson et al., 1946).Subsequent developments made this method obsolete, and more accurate methods using gas-proportional counters and liquid-scintillation counters were developed.For historical reasons, uncalibrated radiocarbon measurements are often referred to a half-life of 5568 years.